This invention relates to inducing anorexia in warm-blooded animals by administering naltrexone orally thereto.
Naltrexone, otherwise known as (-)-17-(cyclopropylmethyl)-4,5.alpha.-epoxy-3,14-dihydroxymorphinan-6-one, is known to be a potent narcotic antagonist which shows considerable promise for the treatment of opiate dependence in man. It can be prepared in accordance with the teachings of Blumberg et al. in U.S. Pat. No. 3,322,950, as well as those of Pachter et al. in Canadian Pat. No. 913,077.
Obesity is a serious problem, leading to, among other things, an increased risk of cardiovascular disease. None of the appetite supressants currently available is completely satisfactory.
Holtzman, J. Pharmacol. Exp. Ther., Vol. 189, pages 51-60 (1974), has shown that naloxone (N-allyl-14-hydroxy-7,8-dihydronormorphinone) suppresses eating in food-deprived rats but not in food-deprived mice. In a subsequent study, Holtzman showed that the fluid intake (sweetened Enfamil) of rats was reduced following subcutaneous administration of naloxone, naltrexone or nalorphine (N-allylmorphine); Life Sciences, Vol. 16, pages 1465-1470. Having observed in the prior study that the anorexigenic activity of naloxone was species dependent, in the latter study, Holtzman questioned whether naltrexone and nalorphine might show similar species specificity.
The oral route for drug administration is the oldest, and it may be the safest, most convenient, most important, most practical and most economical. However, the fact that a drug causes a certain effect when administered by the parenteral route, e.g. anorexia, does not guarantee that the same effect will be obtained at all when the drug is administered by the oral route. Much less is there any assurance that the same effect will be achieved when the drug is administered orally at a reasonable dosage level. Some drugs may be destroyed by digestive enzymes or low gastric pH. Drugs may form complexes with food which cannot be absorbed. In addition, drugs absorbed from the gastrointestinal tract may be extensively metabolized by the liver before they gain access to the general circulation. In some instances, parenteral administration is essential for the drug to be absorbed in active form. Moreover, there may be other disadvantages to administration of a drug by the oral route; e.g. the gastrointestinal mucosa may become irritated by an orally administered drug and cause emesis.
No study directed to the use of naltrexone as an oral anorexigenic agent has been reported heretofore. However, several studies, directed to the safety and efficacy of naltrexone as an oral narcotic antagonist, contain some isolated and contradictory statements relating to the effect thereof on appetite in man. In all of those studies, the appetite effect was reported by the opiate users themselves; it was not observed by any professional personnel, medical or pharmacological.
Shecter et al., Proceedings of the National Association for the Prevention of Addiction to Narcotics, pages 754-765 (1974), report oral administration of naltrexone to 30 male and female subjects, all but 2 of which had been dependent on opiates. One patient, who was receiving 50 mg of naltrexone per day, reported appetite decrease during the naltrexone maintenance period. Another patient, who was receiving a placebo, reported appetite decrease during induction, maintenance, and abrupt withdrawal periods; while yet another patient, who was also receiving a placebo, reported appetite increase during all three such periods. Other patients, who were receiving 125 mg of naltrexone on alternate days of each week, reported both appetite decrease and appetite increase (the number of such patients was not given). No such effect on appetite was reported by the 2 subjects (physicians) having no history of opiate dependence.
Lewis, American Journal of Drug and Alcohol Abuse, Vol. 2, (3-4), pages 403-412 (1975), states, in a preliminary report, that the six symptoms most frequently reported by male opiate-addicted patients, following oral naltrexone induction and maintenance, were sweating, yawning, muscle and joint pain, lack of appetite and lack of sexual desire. Except for lack of appetite, all of those symptoms were present also at the same or higher levels prior to naltrexone induction and maintenance. No dosage level information was given by Lewis. Loss of appetite in opiate-addicted patients who are in withdrawal is, of course, no indication of the anorexigenic effect of a drug in non-addicts.
In a study of "street addicts", "methadone maintenance" patients and "post addicts", Bradford et al., National Institute of Drug Abuse Research, (1976) Monograph Series 9, one patient out of 45 who was receiving naltrexone, and one patient out of 45 who was receiving a placebo, reported loss of appetite. No dosage information was given.